IEEE 802.11 standard defines two operating modes: an ad hoc mode and an infrastructure mode. In the ad hoc mode, two or more STAs can recognize each other and establish a peer-to-peer communication without the need of an AP. In the infrastructure mode, there is at least one AP. The AP and one or multiple STAs it supports are known as a Basic Service Set (BSS), which roughly corresponds to a cell in cellular network environment. A STA uses the AP to access the resources of a wired network, as well as to communicate with other STAs within the same BSS. The wired network can be an organization intranet or the Internet, depending on the placement of the AP. A set of two or more BSSs connected by a distributed system (DS) form an Extended Service Set (ESS), identified by its Service Set Identifier (SSID). If the radio coverage areas of two APs overlap, handoff occurs when a STA moves out of the coverage area of an AP and enters that of another AP.
The handoff procedure involves a sequence of actions and messages exchanged by the STA and neighbor APs, resulting in the transfer of STA's connection from the serving AP to a new AP. During this period, the communication link between the STA and the serving AP is broken, and the STA is not able to send or receive any data packet until establishing a new link with the new AP. So, there is a communication disruption period as illustrated in FIG. 1, which starts from the time the existing communication link is broken to the time when the new link is established. The STA initiates the handoff procedure when it detects that the link quality with the serving AP has degraded below a specific threshold.
As shown in FIG. 1, the communication disruption period is comprised of a scanning process (also called discovery process) and an authentication and re-association process. During the scanning process, the STA needs to switch to each radio frequency (channel) to discover whether there is any AP working on this channel. This scanning process can take up to several hundred milliseconds and occupy over 90% of the whole handoff latency. The authentication and re-association process takes only a few milliseconds to complete.
The channel scanning process can be accomplished in passive or active mode. With passive scanning, STA switches to each candidate channel and listens to periodic beacon frames from APs. An AP uses beacon to announce its presence, its working channel, its BSSID and other parameters for STA's access. The AP broadcasts its beacons periodically (typically every 100 ms). So, to get information about all the APs in a certain channel, the STA has to stay in the channel for at least a beacon period. Comparatively, with active scanning, STA broadcasts Probe Requests in each candidate channel and waits for Probe Responses from neighbor APs working on that channel. An AP sends unicast Probe Response to the STA after receiving the Probe Request. The Probe Response frame carries the same parameters as in the beacon frame. In both cases, after scanning all candidate channels, STA selects the best AP from the records to perform the second process—authentication and re-association.
Due to limited coverage of a BSS, the time for a mobile user to stay in a cell may be on the order of only several minutes, or even a few seconds, depending on its moving speed. Real-time interactive applications have strict quality requirement. For example, VoIP requires its end-to-end delay to be lower than 250 ms, delay variance or jitter lower than 50 ms, and packet loss rate less than 1%. However, with the standard 802.11 protocol, the handoff process cannot satisfy the requirements of real-time interactive applications for the following two reasons:
(1) the communication disruption period is too long (about 500 ms); and
(2) the long communication disruption causes packet loss.
Offering real time handoff is an essential requirement for VoIP and other real time services like video conference. How to provide fast link-layer handoff in WLAN environment is an active research subject, and there are already some related inventions to reduce the handoff latency. Since the scanning process dominates the communication disruption period of a handoff, almost all these inventions attempt to shorten this process. According to the said two modes of scanning process, these inventions fall into two categories: 1) active scanning; and 2) passive scanning.
Active scanning is further categorized as full-scanning and selective-scanning according to the number of scanned channels. Full-scanning is a brute force scheme that probes all the legitimate channels (for example, all eleven channels for 802.11b). Selective-scanning, on the other hand, limits scanning to a subset of legitimate channels. The latency of active scanning is affected significantly by two parameters: the probe count and the probe wait time. Most of inventions using active scanning intent to decrease the probe count. An example is Reference 1 (PCT international publication WO2004/054283A2 by Zhong et al., entitled “System and Method for Performing a Fast Handoff in a Wireless Local Area Network”), which discloses a system and method using a table of pre-configured nearest-neighbor APs to perform a prioritized scanning in the communication disruption period. In Reference 2 (S. Shin, A. Forte, et al., “Reducing MAC layer Handoff Latency in IEEE 802.11 Wireless LANs.” in Proceedings of the Second International Workshop on Mobility Management and Wireless Access Protocols, Philadelphia, USA, 2004), selective scanning and “AP cache” which records the scan results of last scanning are used to realize a link-layer fast handoff. The probe count and the probe wait time are reduced in Reference 3 (M. Shin, A. Mishra, and W. Arbaugh, “Improving the Latency of 802.11 Handoffs using Neighbor Graphs,” in Proceedings of the ACM MobiSys Conference, Boston, Mass., USA, June 2004) by using neighbor graphs and non-overlap graphs. The neighbor graphs construction and probing method is also presented in Reference 4 (US2006/0092883A1). Reference 5 (US2006/0072507A1, entitled “Minimizing Handoffs and Handoff Times in Wireless Local Area Networks”) presents a method, in which the number of channels that are scanned during a handoff is reduced by tracking past user movements within the WLAN.
Some inventions strive to improve the performance of passive scanning. SyncScan in Reference 6 (Ishwar Ramani, and Stefan Savage, “SyncScan: Practical Fast Handoff for 802.11 Infrastructure Networks,” in Proceedings of the IEEE Infocom Conference 2005, Miami, Fla., March 2005) synchronizes the short listening periods at the STA with regular periodic beacon transmission from all the APs. With the knowledge of when the APs on a certain channel will broadcast their beacons, STA can switch to the channel at a particular time and get all broadcasting beacons from these synchronized APs without waiting for a full beacon period. Since it takes very short time to scan a channel, the STA can perform the scanning process before breaking its current connection with its serving AP. The handoff latency is consequently shortened greatly. In Reference 7 (US2005/0047371A1 by Richard L. Bennett, entitled “Passive Probing for Handoff in a Local Area Network”), the serving AP has responsibility to send probe requests to its neighbor APs and inform them of a defined time and a response interval at which they transmit their probe responses. STA is also informed by its serving AP of the defined time, the response interval and the defined channel at which it can hear the probe response from one of its neighbor APs. With the probe responses, the STA can make a decision about when to handoff and which neighbor AP to handoff to. In Reference 8 (Vivek Mhatre, and Konstantina Papagiannaki, “Using Smart Triggers for Improved User Performance in 802.11 Wireless Networks,” in Processing of the ACM MobiSys Conference, Uppsala, Sweden, June 2006), a mechanism is adopted by which STA can hear the beacon from its neighbor APs on the same or overlapping channels with its current channel. Then with a complementary algorithm, the STA can make the right decision which neighbor AP can provide better link quality.
An approach called MultiScan is proposed in Reference 9 (V. Brik, A. Mishra, and S. Banerjee, “Eliminating handoff latencies in 802.11 WLANs using multiple radios: Applications, experience, and evaluation,” in ACM/USENIX Internet Measurement Conference (IMC), Oakland, Calif., October 2005), which relies on double interfaces in each STA to realize seamless handoff. MultiScan nodes use their (potentially idle) second wireless interface to opportunistically scan and pre-associate with alternate APs and eventually seamlessly handoff ongoing connections, while its first interface keeps communication with its serving AP.
A real-time channel scanning mechanism is proposed in Reference 10 (J. Ok, S. Komorita, A. Darmawan, H. Morikawa, and T. Aoyama, “Design and Implementation of Real-time Channel Scanning Mechanism using Shared Beacon Channel in IEEE 802.11 Wireless LAN,” technical report of the Institute of Electronics, Inforamtion and Communication Engineers, Technical Committee on Information Networks (IN2005-208), pp. 305-310, March 2006). In this solution, a shared channel named Beacon-Channel (utilizing channel 14 in the algorithm) is used to eliminate the time-consuming channel scanning. Each AP periodically transmits extended format beacons, called eBeacon, in a Beacon-Channel via an extra interface. As long as a STA has an extra receiver which is tuned to Beacon-Channel, it is able to keep updating eBeacons and tracing the signal quality of neighbor APs.
In all the active scanning methods above, the scanning process keeps in the communication disruption period. That is to say, these methods still conform to the pattern illustrated in FIG. 1. Although being shortened, the channel scanning process still contributes dominating latency for the disruption period. Moreover, with these methods, STA cannot monitor signal qualities of nearby APs continuously, so it may initiate scanning only when the signal with the serving AP has degraded below a threshold, with which connection has to interrupt or endures poor and unsustainable performance, even if there exists a nearby AP with better link quality. Therefore, the STA cannot always choose the best AP to make association with. After scanning, the STA chooses the best AP only according to the one-time sampling result, so the temporary fluctuation of signal can put some influence on the correctness of the AP selection.
SyncScan and the method in Reference 7 can enable STA to monitor the qualities of nearby APs continuously, so that the STA can evaluate the quality of an AP based on average signal quality, and can make choice of the best AP even before the current link turns into poor and unsustainable performance. But both of them require precise synchronization mechanism to enable neighbor APs to send out beacons or probe responses at the right time and to enable STA to hear the beacons or probe responses at the exact moment that neighbor APs send out beacons or probe responses. If the STA, the serving AP, and the neighbor APs are out of sync with each other, beacons from nearby APs will be missed by STA, which will put bad impact on the handoff performance and obstruct the STA from finding the best neighbor AP timely. In large-scale wireless network, it is very difficult to make all APs and STAs synchronized with high time precision. Moreover, to prevent packet loss during scanning process, STA must implement buffering mechanism and send PSM data to AP periodically. It results in significant power consumption in STA.
In order to reduce the co-channel interference, people try to use non-overlapped channels to cover a certain area, such as channel 1, 6 and 11 for 802.11b. It is very different with the assumption presented in Reference 8. Reference 8 assumes there always exist multiple neighbor APs operating in the overlapped channel with the serving AP. Therefore, if there is no neighbor AP operating in the overlapped channel, it is impossible for the STA to find an available AP to connect with. For example, if a STA communicates with its serving AP in channel 1 and neighbor APs operate in channel 6 and 11, the STA will use the standard 802.11 handoff procedure. On the other hand, even if there exist some neighbor APs in an overlapped channel, the STA often can't find the best AP to connect with, since it can only get the information about its neighbor APs on the same or overlapped channel.
Two interfaces in a STA as presented in Reference 9 can make the handoff process completely seamless, but this adds one more apparatus. And, the current reality is that most of the portable terminals are equipped with only one interface. Two interfaces in a STA can also cause more power consuming than a single interface, and the kernel of the STA with two interfaces needs to be modified to make choice which interface should be used for upper layer traffic.
The solution proposed in Reference 10 also requires a STA equipped which two interfaces. The limitations of two interfaces on the STA have been addressed above. Moreover, utilizing channel 14 as the necessary Beacon-Channel is not full compatible with existing 802.11 systems. Further, since channel 14 is the channel allowed for IEEE 802.11b only in Japan, the regions the method can be used is limited.